1. Field of the Invention
The present invention relates to a method of data communication among mobile phones and other communication terminals, a communication server system for controlling the data communication, and a communication terminal.
2. Description of that Art
In recent years, the forms of communication by mobile phones and other communication terminals have become more diversified. In addition to voice communication by line exchanges, new forms of communication such as packet communication utilizing Internet Protocol (IP) networks have begun to be used. For example, Japanese Patent Publication (A) No. 2003-174520 discloses “chat mail” for real time text conversations between communication terminals. “Chat mail” is one of the forms of communication known as “group communication” where previously registered members form a group and communicate with each other in real time by text and other media data.
One form of group communication attracting attention in recent years is the technique called “Push to Talk Over Cellular (PoC)”. In PoC, a plurality of communication terminals form a group is and one communication terminal communicates with many communication terminals” in the group in real time. The sending side communication terminal sends data toward the other plurality of communication terminals in the group, while the receiving side plurality of communication terminals receive the data all together in this form of communication In such real time group communication, communication with each other is possible, but due to the restriction of the bandwidth used, one communication terminal and another communication terminal cannot simultaneously send to each other. For example, when one communication terminal is communicating with the other communication terminal, the other communication terminal can only receive. The other communication terminal cannot send to that communication terminal. The same is true in reverse communication. That is, communication is sometimes carried out by a “half-duplex communication mode”.
When engaging in real time group communication such as PoC by the “half-duplex communication mode”, however, when a sending side communication terminal sends data to a plurality of the receiving side communication terminals all together, even when one of the receiving side communication terminals is unable to communicate due to degradation of the quality of communication on the reception channel etc. and therefore that communication terminal cannot suitably received the data, since the communication is carried out by the “half-duplex communication mode”, that receiving side communication terminal cannot report to the sending side communication terminal it was not able to receive the data normally. As a result, the sending side communication terminal cannot learn of the existence of a communication terminal which cannot receive the data and the sending side communication terminal cannot resend the data to the receiving side communication terminal which cannot normally receive the data, so there is the problem that good group communication is obstructed.
Further, in recent years, as disclosed in Japanese Patent Publication (A) No. 2004-297576, what is being communicated has gone beyond voice and text and now includes still pictures and movies involving large amounts of data. For example, mobile phones mounting cameras and able to transmit images captured by those cameras are known.
When transferring data such as voice, text, and images in real time group communication, the time required until data finishes being received sometimes differs for each user terminal in the group. For example, when mobile phones and other communication terminals communicate wirelessly, the time required until data finishes being received differs for each communication terminal due to the difference of the signal strength and other aspects of the quality of communication. In particular, in the case of data of still pictures and movies involving large amounts of data in comparison with text and voice, this time difference becomes remarkable.
If the time required until data finishes being received differs for each communication terminal, the following disadvantages are liable to occur in group communication of the “half-duplex communication mode” by for example PoC:
(1) When a certain communication terminal (first communication terminal) finishes receiving data, then as a next action transmits new data to a plurality of other communication terminals (second communication terminals), if there is another communication terminal which has not finished receiving the data, the new data transmitted by that first communication terminal will be received in real time at the communication terminals which have finished receiving the previous data among the plurality of second communication terminals, but the communication terminal which has not finished receiving the previous data will end up entering a “Queue state” where it waits for the completion of reception of the previous data. For this reason, a difference ends up occurring in the reception time of new data among the communication terminals. This obstructs good communication.
(2) The first communication terminal sending the data and the second communication terminals ending up finishing receiving the data fast cannot determine at which timing the other receiving side communication terminal will finish receiving the data.
In this way, in real time group communication by the “half-duplex communication mode”, since there was a difference of the time required for data to be finished being received among the plurality of the receiving side communication terminals in the group, there was the inconvenience that neither the sending side communication terminal or the receiving side communication terminals could determine the timing when they could send new data as another action even when they finished receiving data.